1. Field of the Invention
The present invention relates to material processing using pulsed radiation sources. More specifically, it relates to lithography systems using excimer lasers.
2. Description of Related Art
Excimer lasers and other pulsed radiation sources are used in various applications, such as lithographic systems for semiconductor wafer scanners, amorphous silicon annealing for Thin Film Transistor (TFT) processes, and ablation processes for silicon and other materials.
In semiconductor processing, the continual shrink in feature sizes has placed increased pressure on systems to reproduce these smaller features. For example, in lithography the size of features that can be imaged on a semiconductor wafer is often defined as the critical dimension achievable by radiation exposing through a mask (i.e., reticle) and onto a target surface. The critical dimension is governed by many factors; however, one of the dominant factors limiting small critical dimensions is the wavelength of the radiation used to expose the surface. To achieve the lower required wavelengths, lithography systems have turned away from visible light to the smaller wavelengths of ultraviolet radiation produced by excimer lasers. Excimer lasers (excimer stands for excited dimmer) are gas-based lasers comprised of inert and halide gases that generate intense, short, radiation pulses. The halide gas may typically be Fluorine. Various other, typically inert, gases may be used in combination with the Fluorine to produce different wavelengths of ultraviolet light. Some example gases for combination are: Krypton, to produce a 248 nm wavelength; Argon, to produce a 193 nm wavelength; and Fluorine, to produce a 157 nm wavelength. These excimer lasers are generally referred to as KrF lasers, ArF lasers and F2 lasers, respectively.
In generating the necessary power and wavelength, these various excimer lasers undergo a charge/discharge cycle. In this cycle, the laser consumes a relatively constant energy supply, which is stored up until a desired amount of energy is reached. A short pulse in a discharge chamber containing the gases discharges this desired energy, generating a short intense pulse of laser radiation. This charge/discharge cycle results in a maximum frequency at which the excimer laser may operate. Typical maximum frequencies (i.e., pulse rates) are one kilohertz for traditional excimer lasers, two kilohertz for newer excimer lasers, and four kilohertz for emerging technology lasers.
The low spatial coherence of an excimer laser enables illumination of a relatively large area, typically in the form of a rectangular shaped beam. Generally, this rectangular beam may be scanned in one direction across the material to be processed, and then stepped to a new location where the scanning is repeated. An apparatus, typically referred to as a stepper/scanner, performs this process. For example, in a lithography application the stepper/scanner typically scans a rectangular beam across a mask, exposing a photosensitive resist on the surface of a semiconductor wafer at one semiconductor die location. This scanning process may comprise lateral movement of a portion of the scanner apparatus containing the mask and semiconductor wafer such that the mask and semiconductor wafer effectively scan under the stationary beam. In other scanning processes the stepper/scanner, may laterally move the mask in one direction, while moving the semiconductor wafer in an opposite direction, creating an enhanced scanning rate. After scanning a semiconductor die location, the stepper/scanner steps to a new semiconductor die location to repeat the scanning process.
Depending upon the application, any given area of the material may need to be irradiated by as many as hundreds or even thousands of laser pulses as the material is scanned. Because the maximum frequency of pulsed radiation sources is limited, the total throughput for exposing a material is limited. As a result, the total exposure time for the material is limited. Increasing the throughput of a lithography or ablation step in material processing reduces overall processing time and production costs. In order to reduce the overall total exposure time, a method and apparatus to increase the pulsed radiation exposure throughput is needed. Additionally, such an approach may extend the useful lifetime of existing technologies by increasing their throughput while also extending the throughput capabilities of the newest technologies.